In a telecommunications system, when the network recognises that a call has been terminated (normally by one of the parties going "on-hook") the connection is released. This then allows those network elements, which were used by the call to be used by other resources. For example, in a cellular radio environment the radio channel allocated to the call is released so that it can be used by other mobile units to make or receive calls.
For example, in conventional cellular radio systems, when a connection management transaction (for example a speech or a data call) ends, the call control protocol carries out the normal message exchanges to clear the connections. The signalling connections are then also automatically released via appropriate message exchanges from the lower protocol layers.
Similarly, when a mobility management transaction (for example periodic location update) ends, the lower layers are then also automatically released via appropriate message exchanges from the lower protocol layers.
A number of services are available from modern telecommunications systems to assist the maker of a failed call attempt. For example a message may be left for the unavailable user in a voice mailbox. `Call Completion to Busy Subscriber` (CCBS) services are also known in which the system is arranged such that if a caller attempts to call an engaged line it informs that caller when the line becomes free and invites him to make another attempt. When a party ends a call, the signalling and traffic links to that party are released. If one of these services is in use the mobile switching centre must then determine the location of the mobile unit, page it, re-authenticate it, and re-establish the connection to perform the additional service. At busy times, the connection may no longer be available, having been seized by another call. This would lead to the `Call Completion to Busy Subscriber` failing, or appearing to the caller to do so, as the new call attempt would fail. In the case of message services, there is an increased possibility that the unit will have moved out of range, or will have been switched off, before the voice message can be sent, resulting in a delay in receiving the message.